The present invention relates to a cathode material used in secondary lithium ion batteries containing a non-aqueous solution as an electrolyte, a process for the preparation thereof, and a secondary lithium ion battery using the cathode material.
In recent years, with progress of miniaturization of electronic equipments such as cellular phones and portable terminals, higher potential and higher capacity performances are demanded for batteries used in these equipments. Thus, great hopes have been entertained of secondary lithium ion batteries using a non-aqueous solution as an electrolyte which have a large discharge capacity per unit weight and/or per unit volume, and development thereof has been made in various fields.
Layer compounds which can electrochemically undergo lithium intercalation and deintercalation are used as the cathode material in the secondary lithium ion batteries, and composite compounds of lithium and a transition metal as represented by the formula LiMO2 wherein M is a transition metal element, such as LiCoO2, LiNiO2 and LiFeO2, have been used as the cathode material. These composite oxides are obtained usually by mixing a lithium compound such as lithium carbonate or lithium oxide with a transition metal oxide or hydroxide such as nickel oxide or cobalt oxide in a predetermined ratio, and sintering the mixture in air or in oxygen at a temperature of 700 to 900xc2x0 C. for several hours, as disclosed in U.S. Pat. No. 4,302,518 and U.S. Pat. No. 4,980,080.
For the purpose of increasing the capacity or improving the charge/discharge cycling performance, there are proposed LiNixCo1xe2x88x92xO2, which has a combined composition of these composite oxides, as disclosed in Japanese Patent Publication Kokai No. 63-299056, and addition of a trace amount of an element such as Al or Ti as disclosed in Japanese Patent Publication Kokai No. 5-242891. However, among the above cathode materials, a cathode material put to practical use at present is only LiCoO2 that a relatively stable capacity is obtained.
The above-mentioned LiCoO2 composite oxide is often synthesized by a conventional dry powder method because the synthesis is relatively easy, and it has been obtained by dry-mixing a lithium compound such as lithium carbonate, lithium oxide or lithium hydroxide with a cobalt compound such as cobalt oxide or cobalt hydroxide and sintering the mixture at a high temperature of about 900xc2x0 C.
However, the dry powder method has a limit in homogeneous mixing. In particular, dry mixing of a lithium compound having a low density with a transition metal compound having a high density is difficult to achieve homogeneous mixing due to a difference in density. This non-homogeneity of a mixed powder becomes a cause of disorder of cathode material crystal structure, so the mobility of lithium ion in a layer structure of the cathode material is lowered to result in lowering of battery capacity.
Also, since in the disordered portion the layer structure is unstable and the interlayer bonding force is weak, the layer structure is destroyed as the intercalation and deintercalation of lithium ion proceeds, so the disorder contributes to deterioration of the cycling performance. That is to say, the above-mentioned composite oxide obtained by the above-mentioned conventional dry powder method has a capacity much lower than the theoretical capacity and still leave room for improvement.
Thus, in order to achieve homogeneous mixing of respective elements which constitute the cathode materials, there is attempted a wet method wherein the mixing is effected in the state of ions by dissolving a salt of the lithium compound and a salt of the transition metal compound in water to form an aqueous solution. For example, Japanese Patent Publications Kokai No. 5-325966 and No. 6-44970 disclose a process for preparing a cathode material by dissolving salts of transition metal and lithium in a suitable solvent for the mixing by wet methods and sintering the resultant.
In this process, lithium and the transition metal are mixed in the state of ions and accordingly a very homogeneous mixing is achieved in the aqueous solution, but the process has a problem that it is very difficult to obtain a desired homogeneous precursor, because the homogeneity is not kept when removing the solvent such as water, and a segregated salt is formed with a coexisting anion, so the respective components are present separately.
In order to solve this problem, a method for preparing a coprecipitate of a plurality of ions by addition of a suitable precipitant (coprecipitation method) is investigated.
However, the coprecipitation method is very general method and is not suitable for coprecipitation of elements greatly different in chemical properties like in the case of alkali metal ions (lithium ion) and transition metal ions. They precipitate separately and, therefore, it is difficult to achieve a homogeneity of precipitate by this method.
A method wherein a complexing agent capable of forming a composite complex with a cation present in the solution is added (complex method) is also investigated. In this case, the both cations which are herein lithium ion and transition metal ion, form a composite complex and, as a result, a homogeneity of ion mixing in the state of composite complex can be maintained.
For example, Japanese Patent Publication No. 6-203834 discloses a complex method by adding ethylene glycol to lithium salt of acetic acid and a transition metal salt of acetic acid to prepare a complex alcoholate, removing ethylene glycol to form a gel and sintering the gel to obtain a cathode material. Also, Japanese Patent Publications Kokai No. 6-163046 and No. 7-142065 disclose a complex method by subjecting a solution of a salt of a lithium compound, a salt of a transition metal compound and citric acid to a dehydration polymerization to form a gel and sintering the gel to obtain a cathode material.
However, the complex method encounters a problem in a means for removing the solvent from the composite complex If various complexing agents are used, a complex ion wherein a plurality of element ions form a complex can be present at least in the solution, but this state is not always maintained when the solvent is removed, thus resulting in formation of a precursor (gel) with poor homogeneity which is not distinct from the conventional dry powder method.
That is to say, the solvent is gradually removed from the composite complex over a very long time and, as a result, ethylene glycol or citric acid undergoes a dehydration polymerization to form a gel (precursor). The produced gel forms a network which can hold water therein. The precursor is dissolved again in water which has not been able to be removed during the dehydration polymerization or water from air, and forms separate salts with a coexisting anion such as acetic acid radical or nitric acid radical to precipitate. Thus, a deviation in composition generates and the homogeneity achieved in the stage of producing the complex in the solution is impaired after the removal of solvent.
Also, since the cathode materials used in secondary lithium ion batteries are apt to be easily damaged by water, wet methods using the complex method as mentioned above which has a possibility that water remains in the stage of a gel state, are not suitable for the synthesis of the cathode materials.
Further, since the above wet method is a reaction accompanied by gelation, it has a problem of handling that a viscous gel is hard to handle.
Also, since the above-mentioned wet methods require a large amount of a coprecipitaing agent or a large amount of a complexing agent such as ethylene glycol and accordingly a long time is required for the precipitation or the dehydration polymerization, they have a problem that the yield of the precursor is low. Furthermore, since it is required to pass through complicated production steps such as drying under reduced pressure, the methods are not a practical synthesis method for batteries using a large amount of a cathode material.
Spray drying is known as a method for drying a powder. The spray drying is often used for the purpose of preparing particles, but there is a report that the spray drying is applied to a cathode material. For example, J. R. Dahn, U. von Sacken and C. A. Michal, Solid State Ionics, 44, 87-97(1990) discloses a method of the synthesis of LiNiO2 wherein an aqueous solution of LiOH and an Ni(OH)2 powder are mixed to form a slurry, and the slurry is spray-dried to prepare a precursor of Ni(OH)2 powder coated with LiOH, followed by sintering the precursor to give a cathode material. Japanese Patent Publication Kokai No. 2-9722 discloses a method for preparing a manganese oxide powder wherein an aqueous solution of a manganese compound and a lithium compound is formed into a mist by using a ultrasonic humidifier to give a Lixe2x80x94Mn oxide precursor, and the precursor is then sintered to give a cathode material.
However, these spray-drying methods are utilized only for coating onto the surface of particles or for removing solvent, and are not a method which provides cathode materials having excellent performances.
Also, in case of drying in the form of a mist, a mixed solution (containing no complexing agent) obtained from solutions of only raw material components containing a plurality of ions, there arises a problem that the solution is dried eventually in the state that the respective ions are separated.
The present invention has been made in order to solve the above problems, and objects of the present invention are to obtain a cathode material having a homogeneous composition for secondary lithium ion batteries and a process for preparing the same with ease and in a good mass productivity, and to obtain a high performance secondary lithium ion battery using this cathode material.
The first process for preparing a cathode material according to the present invention comprises the steps of obtaining an aqueous solution of a water-soluble salt of lithium, a water-soluble salt of at least one transition metal element selected from Ni, Co, Mn and Fe and a complexing agent capable of forming a complex with said lithium and said transition metal element, removing a solvent of said aqueous solution by spray-drying to give a precursor, and heat-treating said precursor. The cathode material having an excellent homogeneity can be synthesized in a good mass productivity by this process.
The second process for preparing a cathode material according to the present invention is a process that in the first process, the metal element ions in said solution are lithium ion, Ni or Co ion and Mn ion, and the ratio of lithium ion:Ni or Co ion:Mn ion is 1:1xe2x88x92y:y (0.01xe2x89xa6yxe2x89xa60.3). By this process, a cathode material precursor having very homogeneous and stable composition is obtained, and a high performance cathode material is obtained by heat-treating (sintering) this precursor. This cathode material has a small size, and micro-pores open to an electrolyte are formed in the inside and surface of the particles, so not only the surface area (specific surface area) contacting the electrolyte is increased, but also the valency of Ni or Co and the crystalline structure in the cathode material are stable at the time of charge and discharge, thus the battery performances can be improved.
The third process for preparing a cathode material according to the present invention is a process that in the first process, the water-soluble salt is any of a nitrate, a sulfate, a chloride, a fluoride, an acetate and a hydroxide, whereby it is possible to homogeneously mix lithium and the transition metal in the aqueous solution.
The fourth process for preparing a cathode material according to the present invention is a process that in the first process, the complexing agent is any of oxalic acid, tartaric acid, citric acid, succinic acid, malonic acid and maleic acid. By this process, a complex can be easily obtained, so the synthesis can be performed in a good mass productivity.
The fifth process for preparing a cathode material according to the present invention is a process that in the first process, the spray-drying is carried out at an atmospheric temperature of 160 to 220xc2x0 C., whereby the cathode material having an excellent homogeneity is obtained.
The sixth process for preparing a cathode material according to the present invention is a process that in the first process, the spraying pressure of the spray-drying is from 0.5 to 2.0 MPa By this process, the cathode material having a spherical or analogous shape which is advantageous for dense packing of the cathode material in the positive electrode can be easily obtained, and the cathode material having a particle size of 0.5 to 5.0 xcexcm which is suitable for secondary batteries can be obtained without giving damage such as pulverization.
The seventh process for preparing a cathode material according to the present invention is a process that in the first process, the heat treating temperature (sintering temperature) is from 500 to 850xc2x0 C., whereby the lithium component in the cathode material is prevented from subliming or scattering during the sintering, so an ideal cathode material according to stoichiometric ratio can be obtained and the battery performances can be improved by applying it to the positive electrode of batteries.
The first cathode material according to the present invention is one obtained by removing a solvent by spray-drying from an aqueous solution of a water-soluble salt of lithium, a water-soluble salt of at least one transition metal element selected from Ni, Co, Mn and Fe and a complexing agent capable of forming a complex with said lithium and said transition metal element, and heat-treating the resultant. It is excellent in homogeneity.
The second cathode material according to the present invention is a cathode material that in the first cathode material, the metal element ions in the solution are Li ion, Ni or Co ion and Mn ion, and the ratio of Li ion: Ni or Co ion: Mn ion is 1:1xe2x88x92y:y (0.01xe2x89xa6yxe2x89xa60.3), whereby a cathode material precursor having a very homogeneous and stable composition is obtained and a high performance cathode material is obtained by heat-treating (sintering) this precursor. This cathode material has a small size, and micro-pores open to an electrolyte are formed in the inside and surface of the particles, so not only the surface area (specific surface area) contacting the electrolyte is increased, but also the valency of Ni or Co and the crystalline structure in the cathode material are stable at the time of charge and discharge, thus the battery performances can be improved.
The first secondary lithium ion battery according to the present invention comprises a cathode material layer of a positive electrode, an anode material layer of a negative electrode, and a separator provided between said cathode material layer and said anode material layer and retaining a non-aqueous electrolyte containing lithium ion, wherein said cathode material layer contains a cathode material obtained by removing a solvent from an aqueous solution of a water-soluble salt of lithium, a water-soluble salt of at least one transition metal element selected from Ni, Co, Mn and Fe and a complexing agent capable of forming a complex with said lithium and said transition metal element by spray-drying, and heat-treating the resultant, whereby a high performance secondary lithium ion battery is obtained.
The second secondary lithium ion battery according to the present invention is a battery that in the first secondary lithium ion battery, the metal element ions in said solution are lithium ion, Ni or Co ion and Mn ion, and the ratio of lithium ion:Ni or Co ion:Mn ion is 1:1xe2x88x92y:y (0.01xe2x89xa6yxe2x89xa60.3), whereby a cathode material precursor having a very homogeneous and stable composition is obtained and a high performance cathode material is obtained by heat-treating (sintering) this precursor. This cathode material has a small size, and micro-pores open to an electrolyte are formed in the inside and surface of the particles, so not only the surface area (specific surface area) contacting the electrolyte is increased, but also the valency of Ni or Co and the crystalline structure in the cathode material are stable at the time of charge and discharge, thus the battery performances can be improved.